(1) Field of the Invention
The present invention relates to a method of working material with high-energy radiation, wherein a polymer matrix is irradiated with high-energy radiation, in particular with a laser beam.
(2) Description of Related Art
Working material with a laser beam is an established method in industry and is used, inter alia, for welding, cutting, drilling and removing various materials. The variety and complexity of the interacting mechanisms involved in the working of materials with a laser beam are the reason for using a laser beam with process parameters that have been tried out experimentally or in a simulation. When considering how the parameters relate to the adjustable variables of the laser used and the resultant outcome of working, the laser intensity and the time of exposure to the radiation are of particular significance.
It is known that, when working materials by means of a laser, a focused laser beam is directed onto the surface of a workpiece to be worked. The position of the focus is typically chosen such that the distance of the focusing lens makes it possible to obtain the smallest radial extent of the laser beam in relation to the surface of the workpiece. This provides the greatest intensity of the laser at the surface of the workpiece.
As from a certain ratio of the laser power to the extent of the focal spot, the energy density transferred to the workpiece increases abruptly. The coupling in of the laser energy results in phase transformation processes at the surface of the workpiece, making it possible to obtain a specific result of the working.
In the area of surface treatment, the removal of material from the surface of a workpiece by bombardment by means of a pulsed laser beam is referred to as laser ablation.
The energy of the laser photons transferred to the workpiece can lead to the breaking up of chemical bonds, in the case of non-metals it is also possible for short laser pulses to cause a Coulomb explosion. This means that the electrons leave the solid body and some of the remaining positive ions are accelerated out of the surface by Coulomb repulsion.
With laser pulses in the nanosecond range, the energy of the laser leads to heating up of the surface (in the sense of thermal movements of the atoms) during the laser pulse. Since the limited heat conduction allows only a slow energy transfer into the volume, the energy radiated in is concentrated on a very small area. Therefore, the workpiece reaches very high temperatures in this area and abrupt vaporizing of the material can occur. With a high power density of the laser, a plasma of electrons and ions of the material removed may be produced by thermal ionization or ionization induced directly by laser photons, it being possible for the ions of the plasma to be accelerated to energies in excess of 100 eV.
The minimum power or energy density at which ablation is possible (with a given wavelength and pulse length) is known as the ablation threshold. With energy densities above this threshold, the ablation rate increases greatly.
Laser ablation can therefore be used for targeted removal of materials, for example instead of mechanical engraving of hard materials or for drilling very small holes. Alternatively, the material removed may also be used for coating a surface of another workpiece, these techniques being referred to as Pulsed Laser Deposition (PLD) or Laser Transfer Film (LTF).
A disadvantage of laser ablation is that, during and shortly after the laser irradiation, finely thrown up particles of melt, spatter and substances produced by cooling and condensation are often deposited as debris around the working zone. It may be that these decomposition products are removed from the working site by means of process gases. Generally, however, they represent an undesired effect during the working by the laser process and constitute a decisive factor for the quality of the result of the working.
It is usually attempted to minimize these effects by means of laser parameter settings and reactions with process gases. In the case of many materials to be worked, it is possible to use the absorption characteristics to irradiate them with high laser intensities in a short time. The greatest coupling in of the laser energy is usually achieved by the focus of the laser lying on the surface of the workpiece. Energy conversion of the radiation into heat has the effect of forming a heat influencing zone there, in which the thermal effects lead to the desired working results. As a consequence, however, strong thermal processes caused by heat conduction and convection as well as evaporation and plasma formation may produce adverse effects in the surrounding regions.
In the case of poor heat distribution due to a low material-specific coefficient of heat conduction, overheating may occur at boundary surfaces and/or surrounding regions, with the consequence that the material undergoes an undesired structural change. In particular in the case of amorphous and crystalline materials, such as glass, ceramic and crystalline metals such as silicon, it is problematic that this high energy input can lead to adverse effects such as stresses and cracks, which impair the quality of the material to be worked.
A laser beam can be focused in the best possible way if it oscillates in the fundamental mode (TEM00 mode) and its energy distribution follows a Gaussian curve. It is possible by appropriate setting of the focal length by means of the focusing lens to achieve the smallest beam diameters of 40 μm to 120 μm and to direct them onto the surface of the workpiece. Conventionally, the highest pulse power density (J/cm2) of the laser beam is brought onto the workpiece when the focus is set to the surface of the workpiece. If the focal plane of the laser beam does not lie on the surface of the workpiece, the pulse power density may be too low, with the result that the laser beam merely heats up the surface but does not bring about any permanent changes in the material.
If, when focusing on the surface, the absorbed energy exceeds a threshold value, the energy input leads to phase transformations in the material. Although the changes brought about as a result do not necessarily have to be accompanied by a change in the state of aggregation of the material, heating up of the surface of the material causes a temperature field to form in the workpiece. Great temperature gradients lead to thermal stresses, which often remain in the workpiece after the cooling phase as residual stress. Mechanical stresses may, however, also remain in the solid body on account of plastic deformations (for temperatures below 450° C.) in the heating-up phase as a result of thermal expansions. The structural changes forming in the heat influencing zone may, however, also leave optically visible defects behind, such as crater formations, cavities and microcracks.
All known methods for working materials with a laser beam share the common feature that a laser beam is directed onto the surface of a material that absorbs the wavelength of the laser and allows as little light as possible to pass through. This has the effect that the light only penetrates into the material to a small depth and the heat influencing zone is decisively determined by the irradiated surface. The local delimitation of the heat influencing zone also has the effect of restricting the absolute energy transfer to the material. Furthermore, the amount of material that can be removed per unit of time is dependent on how large the heat influencing zone is. On the other hand, it is often not desirable to increase the irradiated surface and to compensate this by increased laser power in order to obtain the necessary pulse power density. The reason for this is that the working of the material with a laser beam is often used precisely when particularly accurate working results are to be achieved, for example in the case of particularly fine or microscopic cuts, bores, marks or the like.
It is therefore the object of the present invention to provide an improved method of working material with high-energy radiation in which the heat influencing zone is increased, without significantly impairing the result of the working.